Mayo Clinic College of Medicine and Science Rochester Rochester, MN
A. K. Bennett, V. Malkov, B. J. Davis, R. Phillips, A. W. Rajkumar, B. J. Traughber Jr, T. J. Patton, M. Veres, D. Spring, M. R. Waddle, J. M. Wilson, J. Kavanaugh, and B. J. Stish; Department of Radiation Oncology, Mayo Clinic, Rochester, MN
Purpose/Objective(s): Ultra hypofractionated stereotactic ablative radiotherapy (SABR) is an emerging alternative to conventionally fractionated post-prostatectomy salvage radiation therapy (SRT) regimens. CT-based online adaptive radiotherapy (oART) is a novel technique that allows rapid re-optimization of dose to account for interfractional changes in target volumes and normal tissues. The DAPPER study investigates the feasibility, safety, and efficacy of combining SABR with oART in the SRT setting. Here we report the initial outcomes from treatment planning and delivery on the DAPPER study. Materials/
Methods: This is a descriptive analysis of logistics and outcomes of oART for the first 25 patients enrolled in the DAPPER study. All patients were treated with a CT-based online adaptive planning platform on a prospective clinical trial. Treatment consisted of 5 fractions of SABR to the prostate fossa (32Gy) with optional inclusion of elective pelvic lymph nodes (LNs) (25Gy) and simultaneous integrated boost(s) to any radiographic or biopsy-confirmed localizable disease in the prostate bed (35.5 Gy) or nodes (37.5 Gy). Contouring, planning and treatment time stamps for each fraction were extracted from the treatment planning software. Student’s t-test was used to compare differences between cohorts with a p = 0.05. Results: A total of 125 fractions from 25 patients were analyzed. Pelvic LNs were treated in 21 (84%) patients, with 9 (36%) and 5 (20%) receiving a boost to involved LNs or gross disease in the prostate bed, respectively. Contour evaluation and adjustment required on average 22 +/- 9 minutes and was significantly greater when including pelvic LNs compared to the prostate fossa alone (24 minutes vs. 16 minutes, p < 0.001). The adaptive radiation plan was selected for 111 of 125 (89%) fractions. Amongst fractions where the adapted plan was selected, 45% were adapted due to improved target coverage of at least 5% over the baseline plan, 10% were adapted to meet organ at risk (OAR) constraints, and the remaining 45% were adapted for a combination of both improved target coverage and OAR sparing. Median treatment time, calculated as time from first cone-beam CT to end of treatment delivery, was 45 +/- 17 minutes. Treatment times were longer for patients treated with pelvic nodal irradiation compared to the prostate fossa alone (47 minutes vs. 34 minutes, p < 0.001). Average treatment times increased with the number of target volumes treated (38 minutes for 1 target vs. 60 minutes for 4 targets, p = 0.002). There is a trend towards greater treatment time for fraction 1 compared to fraction 5 (49 minutes vs. 43 minutes, p = 0.20). Conclusion: These data demonstrate that post-prostatectomy SABR utilizing CT-based daily oART is clinically feasible and allows for improved dosimetry as compared to baseline treatment plans. Oncologic efficacy and clinical toxicity data for patients treated with this technique await further follow up but no events have occurred requiring protocol pause.
